3.1.1 (a,b) The Structure of the Periodic Table
Syllabus
(a) the Periodic Table as the arrangement of elements:
(i) by increasing atomic (proton) number
(ii) in periods showing repeating trends in physical and chemical properties (periodicity)
(iii) in groups having similar chemical properties
What does this mean?
Frankly if you don't already know this at the start of an A level Chemistry course you were ill advised to start the course at all.
To recap, various scientists had tried to arrange the known elements by mass and by chemical properties with limited success.
Until Mendeleev came up with a much more workable model in which he predicted properties of elements yet to be discovered and fitted them into his new table.
We still work with a modified version of his table
But if we still arranged the table by the mass of atoms there would be problems.
Notice that Argon is heavier than Potassium but comes after it.
We could swap them round but then Group 1 would consist of Alkali Metals and Argon which is an unreactive gas.
And Group 0 would consist of Noble Gases and a reactive metal.
For the table to have groups of elements with similar properties it is necessary to reverse them.
Further into the table Te and I are reversed for similar reasons.
So what are they arranged by now?
Mendeleev knew nothing about Atomic Structure.
But if he had known about Atomic Number then he would probably have arranged his elements that way, as we do now.
But simply arranging elements by Atomic Number would result in a long list, rather than a table.
So we also arrange the table by electronic configuration.
This puts all elements with electronic configuration ending with 1 in Group 1 and all elements with electronic configuration ending with 2 in Group 2 etc
It also means that as we go across each period (row) the elements change from metals to non-metals in a fairly uniform way - this is called Periodicity.
So the table we have today is different to Mendeleev's because of discoveries made after his death.
But arranging it by chemical properties rather than mass was a good first step.
We refer not only to Groups (columns) and Periods (rows) but also to Blocks.
Anything in the s-block has its outermost electron in an s-orbital.
Anything in the p-block has its outermost electron in an p-orbital - and so on
The problem with this is Period 1.
We conventionally put He in Group 0 because it is unreactive like Noble Gases.
But it really ought to be in Group 2 because its outermost electron is in an s-orbital and it is the second element in Period 1
But it is nothing like Group 2 elements.
Similarly, H is often placed in Group 1 due to its electronic structure but is nothing like an Alkali metal.
It is an s-block element, however.
Generally it is left floating randomly apart from other elements because there's really nothing else quite like it.
So clearly our Periodic Table isn't perfect but its better than a lot of the alternative suggestions.
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